Mistress Tiffany was exasperated because she felt like her slave did not understand her. Or if he understood her, he ignored her. The mistress wanted to punish him and make sure that he understood all she said and he acted on the what she told him. So she used her muddy shoes to do it. She forced him to lick the muddy boots and that served as a lesson to him.
When this mistress noticed that her farmhand was doing the wrong thing on her crops, she ran out of the house with only her bikini and her rubber boots. She did not care how she looked to him and as she stepped on mud on her way to him, she punished him with the same muddy shoes as she stomped on him to make him understand there were no shortcuts and he had to do the right thing.
Mistress Natasha was angered by her boyfriend. He had called her another girl's name and she hated it. It made her feel like she was thinking about another woman when with her and that he might be cheating on her. So she punished him using her muddy shoes. She made him lick her soles and she did other crazy things to him to warn him never to cheat on her.
Goddess Nika was new to domination and torture. She was not comfortable doing the crazy things she had seen other mistresses do. So she sought to start in a small way. And this she did by using her boots to stomp on this loser. She made sure the boots were dirty before she asked him to lick them and then used them to trample and make him dirty. It was a good start for her and she planned to build on it.
Lady Shay had beef with this loser and she made sure he learned his lesson the hard way. She had had enough of him and she chose to punish him cruelly. The mistress went ahead and she made the loser lick her muddy shoes. She did not care how unhygienic it was. When he was done licking her boots, the mistress turned on him and she trampled him as painfully as she could.